The transmitter and receiver filters in wireless communications transmitters, in particular those in cellular phones, require filters with a precisely defined transfer function. In particular, such a filter should have a sufficiently wide transmission band with low insertion loss and steep edges. The suppression of spurious signals outside the transmission band, that is selectivity, should be sufficiently high.
There are other problems associated with the actual construction of suitable filters. Normally, the selection of the substrate material for the surface acoustic wave filter is determined and/or influenced by the desired temperature coefficients and the volume of the electro-acoustical coupling. Also, increasing miniaturization forces a limitation of the available chip area. This, in turn, often means that a desired transfer function for the filter on a limited chip area can only be achieved by using a certain filter technology. Since the selection of the substrate also determines the coupling coefficient, the possibility to influence transducer impedance is low. However, the selection of a suitable filter impedance is very important for the filter's circuit environment and in particular for certain applications. For example, too low filter impedance in a converter with an open collector output results in increased power consumption and, consequently, in increased battery use and shorter operating time in equipment independent of wall power too.
In order to set a desired transfer function, the interdigital transducer must be weighted. Overlap weighting results in reduced overlap time, entailing increased diffraction and resultant transfer losses. In withdrawal weighted transducers, a number of locally divided small overlaps—like in overlap weighting—are replaced by overlaps whose excitation output is uniform throughout the entire transducer. This results, however, in an approximation error, which causes, especially in transducers with large time-bandwidth products, the response of the withdrawal-weighted transducer to deviate substantially from the desired transfer function at continuous weighting.
Another problem is high performance, which, it is true, results in optimal transfer characteristics in connection with narrow tolerance adapter components, though it makes these characteristics sensitive to fluctuations in the adapter components. In particular, broadband filters with high edge steepness show high performance because the actual transducer admittance component of these filters is small.
To increase transducer impedance on a given substrate material, the aperture can be decreased. However, if this is done, surface wave diffraction is increased, thus lowering the achievable edge steepness. Transducer impedance can also be adjusted by utilizing different phase relationships of overlapping partial waves. However, this cannot be done without influencing the reflectivity of SPUDT cells, which are increasingly used in filters. Another possibility to increase transducer impedance consists of lowering the sampling rate. The result of this procedure, however, is to excite higher harmonics, thus worsening selection in the stop band. In withdrawal-weighted transducers, a reduced sampling rate also makes it more difficult to precisely approximate the desired weighting function.
To decrease the approximation error of the withdrawal weighting, it can be combined with overlap weighting, which in turn, however, results in increased diffraction. Using voltage weighting, that is capacitive voltage division, excitation can be reduced in the transducer's subsections without shortening overlap length, thus reducing the influence of diffraction. In this manner, sampling rate can be increased and approximation error lowered in withdrawal-weighted transducers. However, the voltage weighting for the capacitive voltage division requires an additional transducer used only as capacitance. But this requires additional chip area and generates transfer losses.
The frequency characteristics of a filter with high performance transducers can only be achieved with very precise and consequently costly adapter components with low tolerances. An increase in tolerance is only possible if the ripple factor of the transmission band is set even lower and the edge steepness even higher, so that even the desired requirements can still be met despite fluctuating electrical adjustment. However, the above always entails a larger time-bandwidth product and is can only be achieved on an enlarged chip area.